1. Technical Field
This invention relates generally to distributed circuits and, more particularly, to an active load architecture for distributed circuit gate transmission line termination.
2. Discussion
Distributed circuits such as distributed amplifiers, distributed power dividers, and distributed power combiners are commonly employed in various applications, especially broad band communication and radar systems operating at microwave frequencies. Distributed circuits used in these applications often consist of a number of amplifying sections, typically periodically spaced active devices, which are coupled between electrically short, high impedance input and output transmission lines. Impedances in the input and output transmission lines function in combination with internal capacitances of the active devices to produce a desired characteristic transmission line impedance.
This circuit architecture requires a broadband radio frequency (RF) termination on the distributed input and output transmission lines in order to effect matching and thereby prevent reflection of an applied pulse. Transmission line matching is achieved by connecting the transmission line to ground through a terminating impedance substantially equal to the line's characteristic impedance. distributed transmission line termination is made with passive resistive and/or reactive elements. Although low reflection terminations can be realized using these elements, they typically generate substantial thermal noise power which appears on the output of the device to degrade the noise figure of the device. The adverse effect of this termination thermal noise power further increases at low frequencies due to incomplete cancellation of the reverse traveling wave in the output circuit.
It would therefore be desirable to provide a distributed gate transmission line termination for these devices which is both low in reflection and in noise power.